In the field of taste, our focus is the isolation and characterization of genes encoding taste receptors and using these to mark the cells, define the corresponding signaling pathways, dissect receptor specificity, generate topographic maps, and trace the respective neuronal connectivity circuits. This work has identified and characterized two families of G-protein coupled receptors, T1Rs and T2Rs, that are expressed in distinct subsets of taste receptor cells and that include functionally validated sweet, amino acid and bitter taste receptors. In addition, we have shown that the TRP-ion channel PKD2L1 is selectively expressed in sour sensing cells. We have also developed a number of genetically engineered mouse lines that have had a major impact in our understanding of how sweet, bitter, sour, salty and umami taste are encoded at the periphery. In this reporting period, we (in our collaboration with Charles Zuker and his group at UCSD) have continued to focus effort on understanding the details of taste coding at the periphery with work concentrating on using molecular genetic techniques to uncover details of sour, salt and unconventional tastants. We have demonstrated that sour cells also mediate taste responses to carbon dioxide via the extracellular enzyme carbonic anhydrase IV (CA-IV). Knockout mice lacking this enzyme have severely compromised responses to carbonation. The taste of carbonation requires CA-IV function and is strongly inhibited by membrane impermeable inhibitors of carbonic anhydrase. Since CA-IV is specifically tethered to the surface of sour-sensing cells, and thus ideally poised to provide a highly localized acid signal to the sour taste receptor cells, it is likely that extracellular protons act as the relevant signal in the taste of carbonation. We have also investigated the biology of salt sensation and have demonstrated that the sodium channel ENaC is required for mice to be able to discriminate sodium containing salts, to taste low concentrations of salt and to show attractive responses to salt. We have also used cell based assays to demonstrate that the cells with selective responses to sodium chloride are distinct from those that respond to higher concentrations of a number of different salts. In the field of olfaction, our focus has been on the development of methods whereby we can control the expression of odorant receptors. In mice, the odorant receptors are encoded by a family of more than a 1000 genes. A fundamental feature of the mammalian olfactory system is that each olfactory sensory neuron expresses just a single member of this vast family of genes. However, the details of the control of odorant receptor gene expression remain unexplained. In a collaborative project with Leonardo Belluscio, we have demonstrated new aspects of regulation that contribute to the control of odorant gene expression and have devised a system that can reliably generate mice expressing a single odorant receptor in the vast majority of olfactory sensory neurons. Previous studies from other groups have shown that odorant receptors play a key role in establishing a chemotopic map in the olfactory bulb by controlling the precise location where the primary sensory neuron makes synaptic connections with secondary neurons. Again the details of this process remain unknown and are one focus of our research. We are also using various approaches to manipulate odorant receptor expression to investigate the role these receptors play in establishing the connectivity of olfactory sensory neuron in the olfactory bulb.